It is an object of the present invention to provide a gate type antenna device which is preferably used in an IC-integrated media reader or reader/writer for supplying power and transmission data to an IC-integrated medium, e.g., a non-contact IC card and acquiring reception data in the form of load fluctuations from the non-contact IC card.
For purposes such as authentication and sorting of destinations of baggage, it is becoming common to use IC-integrated media having ID codes such as IC cards and IC tags and IC-integrated media readers and IC-integrated media readers/writers for authenticating ID codes of such IC-integrated media. When an IC-integrated medium enters a communication range of an antenna of an IC-integrated media reader, an electromagnetic wave from the antenna generates an induction voltage at an antenna of the IC-integrated medium, and the voltage is rectified into power and transmission data which are then supplied to the IC. In the IC-integrated medium to which the power has been thus supplied, according to data read from a memory incorporated therein, a switch is turned on/off according to 1s and 0s in the data by, for example, a modulation circuit which is formed by a load resistance and a switch and which is connected to the antenna. The switching on/off results in a fluctuation of a load applied to the antenna of the IC-integrated medium, and the load fluctuation is transmitted to the antenna of the IC-integrated media reader or reader/writer as a signal. The transmitted signal is demodulated by the IC-integrated media reader or reader/writer to allow execution of authentication of the ID code (for example, see JP-A-1-126785 and JP-A-2000-163523).
In this connection, there is demand for capability of authenticating ID codes of IC-integrated media, for example, by passing baggage or commodities to which the IC-integrated media are attached through a baggage gate at an airport or an entrance or exit of a store. The capability of authenticating ID codes of IC-integrated media is also desired for purposes like checking of the inventory of a commodity which is placed in a commodity showcase and to which an IC-integrated medium is attached. Under the circumstance, it is desired to use an antenna having stable characteristics to prevent erroneous reading and writing. For this reason, antennas which are fed with very high power and which radiate strong electromagnetic waves are used to allow data to be transmitted and received to and from IC-integrated media with high reliability.
In the related art in which a gate antenna device having a plurality of antennas is supplied with power at all of the antennas as disclosed in Japanese Patent Laid-Open No. JP-A-2003-258545, a problem has arisen in that power consumption increases and in that correction of phase shifts of a reception signal on the different antennas is required especially in communicating with a radio communication medium for mutual data communication.
However, the strength of a leakage electric field from induction-type read/write communication facility is limited to a distance of 10 meters by the Radio Law, and the use of an antenna that supplies high power in the excess of the limit is restricted. For this reason, a limit has been placed on efforts toward increases in power fed to an antenna for reliable communication with an IC-integrated medium. The term “IC-integrated media” implies not only IC media such as IC tags, IC cards, ID tags, and ID cards but also various other media which allow radio communication on a non-contact basis.
An increase in the level of power fed to an antenna results in a reduction of a communication range, which has led to a problem that communication with an IC-integrated medium becomes unsatisfactory.
A multiplicity of antennas may be provided in order to expand the range of communication with an IC-integrated medium. When a multiplicity of antennas is provided, however, a problem arises in that there will be a corresponding increase in power consumption, and this is disadvantageous for construction of a system for which low power consumption is desired. 1. Field of the Invention